You can save the map as an image or PDF and then use other tools to mark it up.

You could be excruciatingly clever and hand draw lines on an image which you use as the background image for the map.

MindManager on the Tablet PC allows handwriting in topics and a sketch per topic. By using Floating Topics you could free draw in the space between topics but the content is inside a rectangular box which cannot overlap other objects. You can turn on the Tablet PC functionality in Windows Control Panel and use a graphics tablet with a standard PC.